The treatment of human diseases through the application of nucleotide-based drugs such as DNA and RNA has the potential to revolutionize the medical field (Anderson Nature 392(Suppl.):25-30, 1996; Friedman Nature Med. 2:144-147, 1996; Crystal Science 270:404-410, 1995; Mulligan Science 260:926-932, 1993; each of which is incorporated herein by reference). Thus far, the use of modified viruses as gene transfer vectors has generally represented the most clinically successful approach to gene therapy. While viral vectors are currently the most efficient gene transfer agents, concerns surrounding the overall safety of viral vectors, which include the potential for unsolicited immune responses, have resulted in parallel efforts to develop non-viral alternatives (for leading references, see: Luo et al. Nat. Biotechnol. 18:33-37,2000; Behr Acc. Chem. Res. 26:274-278, 1993; each of which is incorporated herein by reference). Current alternatives to viral vectors include polymeric delivery systems (Zauner et al. Adv. Drug Del. Rev. 30:97-113, 1998; Kabanov et al. Bioconjugate Chem. 6:7-20, 1995; each of which is incorporated herein by reference), liposomal formulations (Miller Angew. Chem. Int. Ed. 37:1768-1785, 1998; Hope et al. Molecular Membrane Technology 15:1-14, 1998; Deshrnukh et al. New J. Chem. 21:113-124, 1997; each of which is incorporated herein by reference), and “naked” DNA injection protocols (Sanford Trends Biotechnol. 6:288-302, 1988; incorporated herein by reference). While these strategies have yet to achieve the clinical effectiveness of viral vectors, the potential safety, processing, and economic benefits offered by these methods (Anderson Nature 392(Suppl.):25-30, 1996; incorporated herein by reference) have ignited interest in the continued development of non-viral approaches to gene therapy (Boussif et al. Proc. Natl. Acad. Sci. USA 92:7297-7301, 1995; Putnam et al. Macromolecules 32:3658-3662, 1999; Lim et al. J. Am. Chem. Soc. 121:5633-5639, 1999; Gonzalez et al. Bioconjugate Chem. 10:1068-1074, 1999; Kukowska-Latallo et al. Proc. Natl. Acad. Sci. USA 93:4897-4902, 1996; Tang et al. Bioconjugate Chem. 7:703-714, 1996; Haensler et al. Bioconjugate Chem. 4:372-379, 1993; each of which is incorporated herein by reference).
Cationic polymers have been widely used as transfection vectors due to the facility with which they condense and protect negatively charged strands of DNA. Amine-containing polymers such as poly(lysine) (Zauner et al. Adv. Drug Del. Rev. 30:97-113, 1998; Kabanov et al. Bioconjugate Chem. 6:7-20, 1995; each of which is incorporated herein by reference), poly(ethylene imine) (PEI) (Boussif et al. Proc. Natl. Acad. Sci. USA 92:7297-7301, 1995; incorporated herein by reference), and poly(amidoamine) dendrimers (Kukowska-Latallo et al. Proc. Natl. Acad. Sci. USA 93:4897-4902, 1996; Tang et al. Bioconjugate Chem. 7:703-714, 1996; Haensler et al. Bioconjugate Chem. 4:372-379, 1993; each of which is incorporated herein by reference) are positively-charged at physiological pH, form ion pairs with nucleic acids, and mediate transfection in a variety of cell lines. Despite their common use, however, cationic polymers such as poly(lysine) and PEI can be significantly cytotoxic (Zauner et al. Adv. Drug Del. Rev. 30:97-113, 1998; Deshmukh et al. New J. Chem. 21:113-124, 1997; Choksakulnimitr et al. Controlled Release 34:233-241, 1995; Brazeau et al. Pharm. Res. 15:680-684, 1998; each of which is incorporated herein by reference). As a result, the choice of cationic polymer for a gene transfer application generally requires a trade-off between transfection efficiency and short- and long-term cytotoxicity. Additionally, the long-term biocompatibility of these polymers remains an important issue for use in therapeutic applications in vivo, since several of these polymers are not readily biodegradable (Uhrich Trends Polym. Sci. 5:388-393, 1997; Roberts et al. J. Biomed. Mater. Res. 30:53-65, 1996; each of which is incorporated herein by reference).
In order to develop safe alternatives to existing polymeric vectors and other functionalized biomaterials, degradable polyesters bearing cationic side chains have been developed (Putnam et al. Macromolecules 32:3658-3662, 1999; Barrera et al. J. Am. Chem. Soc. 115:11010-11011, 1993; Kwon et al. Macromolecules 22:3250-3255, 1989; Lim et al. J. Am. Chem. Soc. 121:5633-5639, 1999; Zhou et al. Macromolecules 23:3399-3406, 1990; each of which is incorporated herein by reference). Examples of these polyesters include poly(L-lactideco-L-lysine) (Barrera et al. J. Am. Chem. Soc. 115:11010-11011, 1993; incorporated herein by reference), poly(serine ester) (Zhou et al. Macromolecules 23:3399-3406, 1990; each of which is incorporated herein by reference), poly(4-hydroxy-L-proline ester) (Putnam et al. Macromolecules 32:3658-3662, 1999; Lim et al. J. Am. Chem. Soc. 121:5633-5639, 1999; each of which is incorporated herein by reference), and more recently, poly[α-(4-aminobutyl)-L-glycolic acid]. Poly(4-hydroxy-L-proline ester) and poly[α-(4-aminobutyl)-L-glycolic acid] were recently demonstrated to condense plasmid DNA through electrostatic interactions, and to mediate gene transfer (Putnam et al. Macromolecules 32:3658-3662, 1999; Lim et al. J. Am. Chem. Soc. 121:5633-5639, 1999; each of which is incorporated herein by reference). Importantly, these new polymers are significantly less toxic than poly(lysine) and PEI, and they degrade into non-toxic metabolites. It is clear from these investigations that the rational design of amine-containing polyesters can be a productive route to the development of safe, effective transfection vectors. Unfortunately, however, present syntheses of these polymers require either the independent preparation of specialized monomers (Barrera et al. J. Am. Chem. Soc. 115:11010-11011, 1993; incorporated herein by reference), or the use of stoichiometric amounts of expensive coupling reagents (Putnam et al. Macromolecules 32:3658-3662, 1999; incorporated herein by reference). Additionally, the amine functionalities in the monomers must be protected prior to polymerization (Putnam et al. Macromolecules 32:3658-3662, 1999; Lim et al. J. Am. Chem. Soc. 121:5633-5639, 1999; Gonzalez et al. Bioconjugate Chem. 10: 1068-1074, 1999; Barrera et al. J. Am. Chem. Soc. 115:11010-11011, 1993; Kwon et al. Macromolecules 22:3250-3255, 1989; each of which is incorporated herein by reference), necessitating additional post-polymerization deprotection steps before the polymers can be used as transfection agents.
There exists a continuing need for non-toxic, biodegradable, biocompatible polymers that can be used to transfect nucleic acids and that are easily prepared efficiently and economically. Such polymers would have several uses, including the delivery of nucleic acids in gene therapy as well as in the packaging and/or delivery of diagnostic, therapeutic, and prophylactic agents.